| W-Cu composites are usually fabricated by infiltration-sintering porous tungsten compacts with molten copper or by liquid-phase sintering W-Cu powder compacts. However, the mutual insolubility between tungsten and copper results in the very poor sinterability of W-Cu powder compacts. It is now known that using powder with fine particle size and homogeneous distribution of ingredients can improve the sinterability of a powder compact, especially in W-Cu system, for which the dominant sintering mechanism is particle re-arrangement during the liquid-phase sintering. In the thesis, ultra-fine W-Cu powders with 30 weight percents of Cu were prepared by several routes. The property and micro structure of the resulting powders was studied, with emphasis on the sinterability of different W-Cu powders, and the effects of properties of W-Cu powders on the microstructure, the mechanical and physical properties of the sintered W-Cu parts.In chapter 3 of this thesis, a novel process, namely homogeneous precipitation by evaporating ammonia was employed to prepare W-Cu powder. In which aqua ammonia was added into a liquor solution containing natrium tungstate and copper chloride to form a complex solution. The solution was then heated to remove ammonia, resulting in a precipitate contains Cu2WO4(OH)2 and CuW04 2H2O. The precipitate was then washed, dried and dehydrated in the air, followed by reducing in H2, and W-Cu powder was finally obtained. For comparison, W-Cu powder was also prepared by reduction the WO3-CuO mix obtained by ball milling the tungsten and copper oxide together. The test results have showed that the W-Cu powder produced by the homogeneous precipitation process has a near spherical shape, a particle size ranging from 50 to 200nm and a uniform distribution of W and Cu gradients. Contrarily, W-Cu powder prepared by reducing the mechanically mixed WO3-CuO oxides has a particle size ranging from 0.1 to 1 m with irregular particle shape and non-uniform distribution of W and Cu composition.In chapter 4, W-Cu powder compacts were fabricated from the homogeneousprecipitation powder and W-Cu powder prepared by reduction the mechanically mixed WO3-CuO powder. The compacts were then sintered in H2 atmosphere at temperature ranging from 1050 to 1250 C for different time. The density of the sintered compacts was measured using the Archimedes method. The microstructure of sintered body was observed by scanning electron microscope (SEM). The mechanical and physical properties of the obtained W-Cu composites were also tested. It was found that the property of the starting W-Cu powder has a great influence on the sintering densification of the powder compacts. W-Cu composites prepared from the precipitation W-Cu powder have higher sintered density, better mechanical and physical property, more homogeneous and finer microstructure than those prepared from W-Cu powder by the balling-reduction method.In chapter 5, a combination of homogeneous precipitation and ball-milling process, namely the mechano-thermochemical process was employed to prepare ultra-fine W-Cu powder. It was found that ball milling the precipitates and the reduced W-Cu powder can further reduce the particle size and improve the distribution of W and Cu ingredients in the final W-Cu powder. This results in both the increase of sintered density and property, and the improvement of microstructure of the W-Cu composites.
|
PDF Full Text Request